Resolution

Discussion in 'Photography Beginners' Forum' started by e999sam, Feb 15, 2007.

  1. e999sam

    e999sam TPF Noob!

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    What determines the resolution of an image.
    When checking the resolution of 3 different images taken by 3 cameras.
    Image 1 taken with a 3.1 mega pixel camera = 230dpi
    Image 2 taken with a 4.1 mega pixel camera = 75dpi.
    Image 3 taken with a 10.2 mega pixel camera = 240dpi.
     
  2. Torus34

    Torus34 No longer a newbie, moving up!

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    OK. Here we go. Hang on tight!

    For digital rigs (as opposed to film), the resolution of an image can be discussed at several stages.

    First, the sensor chip in the camera itself. This is the 'megapixel' number and indicates the number of individual 'spots' on the sensor. It is the number you get if you multiply the number of pixel rows on the sensor by the number of pixel columns.

    Next, you can also consider the size of the digital file which contains the picture information. This may be 'compressed' in one way or another to make the file smaller. It is not unusual to find that the size of a jpeg file of a picture, especially after editing, is considerably smaller than the megapixel number of the camera which was used to take it.

    And then there is the image of the picture on the CRT or flat screen of your 'puter. The resolution of the screen image in pixels is often far lower than the megapixel count of the camera sensor which took the picture or the pixel count in the file. Several pixels in the file are melded together to form one pixel on the 'puter screen. This is why you will often see an improvement in definition as you enlarge a print on the screen - fewer pixels are 'melded' per screen pixel and the screen image begins to approximate the resolution available in the picture digital file. In fact, you can enlarge an image on the screen to the point where one file pixel will be spread across several screen pixels. At this stage, definition [resolution] will begin to decrease again.

    Finally, we come to the print. Here, we abandon the concept of megapixels and use another term - dots per inch, or dpi. Printers form an image by printing one line at a time. This line is formed from individual dots. The 'fineness' of the line of dots is stated as dots per inch. The processing 'puter determines how many pixels in the image will be grouped together to form one dot in a print. It is possible to relate the dpi in a print to the megapixels of the camera, but it must take into account the 'fineness' of the printing and the size of the print itself.

    Hope this helped. Now, you want to know about film resolution? ;)
     
  3. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    dpi isn't resolution, it's the scale at which the file is printed. If you have a photo that is 100x100 pixels resolution, and you printed it at 10 dpi, it would be a 10"x10" print. If you printed it at 100 dpi, it would be a 1"x1" print.

    The photographer may be able to adjust dpi in some cameras, and it can always be done in graphics editing software.
     
  4. e999sam

    e999sam TPF Noob!

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    If I preview a RAW image in Adobe Bridge at the bottom left under the tab metadata resolution is 240 dpi. If I then save this as a JPEG or TIFF image the resolution drops to 72 dpi. If I save as a psd image it stays at 240. Does this mean there would be a difference in size or quality when printed out.
     
  5. EOS_JD

    EOS_JD TPF Noob!

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    This is not correct.

    You are mistaking dpi for ppi. If you print the 100px x 100px image at 10PPI then you'll get a 10" x 10" print. The dpi setting will determine the quality of the printers output. Generally we print at much higher resolutions that the res of the image. See below.
     
  6. markc

    markc TPF Noob!

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    In a perfect world, DPI and PPI would be separate and used correctly. Unfortunately Photoshop has used DPI in place of PPI, so we have to live with that. PPI (or Photoshop DPI) is as Matt stated. True DPI is solely a printer setting and determines how many dots of ink go into an inch. I blame Adobe for ignoring this distinction and making it difficult to talk about. Many other editors have followed suit.
     
  7. EOS_JD

    EOS_JD TPF Noob!

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    There's always an aweful lot of confusion and bad advice given in relation to resolution so let me try and make this as easy to understand as possible.

    For a start dpi only relates to the printers output (or scanning resolution) - nothing else. Many people make the mistake of using dpi instead of ppi (see above).

    Images are made up of millions of pixels. So in relation to resolution the term should be ppi (pixels per inch).

    The resolution coming out of your camera i.e 72ppi/240ppi etc., matters not a jot until you come to printing. The only thing that matters is the number of pixels.

    My 20D produces a 3504x2336 pixel image. Whether it comes of my camera at 1ppi or 1000ppi the image is still a 3504 x 2336 image. Try converting in RAW at different ppi values and you'll see the total number of pixels stays the same but the print size falls - explained below.

    Now when it comes to printing there are a couple of very easy ways to work out the unknown variable like print sizes or resolution.

    1. Say you know the size of print you want (say a 10"x8") and you want to print it at 300ppi on your printer. You would need an image of 3000pixels x 2400 pixels in order to do this. If you have more pixels you can print at a higher resolution (or as many do - crop the image to the required pixel count). Quality from printing at higher than 300ppi is unlikely to be seen by the human eye so 300ppi seems to be the preferred quality for small images. To be honest you can actually print at around 240ppi without any noticable drop in quality in a small image like a 10x8 and for larger prints your resolution can drop even lower with no noticable drop in quality (at normal viewing distances).

    2. Say you know the number of pixels you have (the camera default for instance) and you want to print a large 18" x 12" print. At what res will this print out at without resampling the image.

    The easy formula is Resolution= pixels/print size
    20D image is 3504 x 2336
    so:

    3504/18 = 194.666ppi and
    2336/12 = 194.66ppi

    Sp roughly 196ppi which contrary to popular belief, will provide a high quality image that will look fantastic from normal viewing distance.

    Because viewing distance will be further than that of a 6x4, the ppi can be a lot lower for larger images. (look from a distance at a billboard then look close up and you'll see what I mean). By printing these larger images at 300ppi all you do is increase the image size (greatly) and at the normal viewing distance you will not see a difference in the print! I print my 19"x13" prints at just under 180ppi.

    You can also add a third equation to work out how many pixels you need for an image. If you want a 12x8 print at 300ppi you need 12x300=3600 and 8x300=2400. So you need an image of 3600x2400 (just outside the normal range of an 8Mp camera.

    You can increase the number of pixels in your image (use the resample button in photoshop's image resize menu). With resampling switched on, you can increase the pixel content of your image. I try not to resample if I can help it. Even printing a 6x4 @ 240ppi or slightly less will give perfectly acceptable prints from a decent printer.

    As I say, I try not to resample too much because all you are doing is either adding information that is not there in the first place (adding pixels or upsampling) or deleting pixel information (downsampling).

    There's so much misinformation on this subject around the web it's amazing how anyone gets to grips with this stuff.

    Printers print in dots (dpi) and my R2400 can print at about 5670dpi. Now no matter the pixel resolution of my image I can print it at this high setting. So my 72ppi file that is 6x4 will still print at 6x4 but my printer will print it at 5760dpi but it'll still look crap because the 72ppi is not a high enough image resolution to get a good print.

    dpi is only for printing and scanning and should not be confused with images which are made up of pixels.

    Does this make sense? Hope I can help those who are struggling to understand this. Your use of digital images will improve when you understand the basic concept.

    Here's a pretty good link

    http://www.steves-digicams.com/techc...uary_2005.html

    Cheers
    Jim

    PM me if you have any queries as this is my first post here.
    Regards
    Jim
     
  8. EOS_JD

    EOS_JD TPF Noob!

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    Photoshop uses PPI quite correctly?!

    It's others who use dpi that are making the error. I believe CaptureOne calls it dpi.

    It's a lack of knowledge from many users around the world that has got everyone talking dpi instead of ppi.
     
  9. EOS_JD

    EOS_JD TPF Noob!

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    See my post above - these numbers mean nothing until you actually come to print and are easily edited to get the resolution / print size you want. If you have trouble uinderstanding let me know.

    Cheers
    Jim
     
  10. ksmattfish

    ksmattfish Now 100% DC - not as cool as I once was, but still

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    None of the Canon or Adobe software I use uses the term "ppi". It's all listed in "dpi". I stand by my answer for real world application. take it up with Canon or Adobe if you want them to use different terminology.
     
  11. EOS_JD

    EOS_JD TPF Noob!

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    those individual spots equate to pixels on your digital image. This is the most important number when determining anything regards the size or resolution of your image.

    This has no bearing on resolution at all. This is about file compression.

    With regards screen resolution, this is why we resample web images down to show your whole image on a typical screen. If you have a 1024 x 768 screen res, to view the whole of a typical image you would need a resolution at less than 1024 pixels (horizontal res) and/or 768 pixels vertical res). So a typical web web image of 600 x 400 the whole image will be seen at 1 pixel on the image= 1 pixel on the screen and should take up approx 50% of the screen.

    If you try to put a larger image than 1024 x 768 pixels on your screen you would need to view at less than 100% to see all of the image. Never heard the word "melded" but the pixels are too small to be seen as one pixel and they do sort of merge together digitally).

    The "line" you speak of is determined by the width of the print head. the print head has holes in it that spurts out the ink onto the paper at a setting you set in the print driver. Highest on y R2400 is something like 5760dpi.

    The rest of what you say regards printing makes no sense to me. It is not possible to relate the dpi in a print to the megapixels in a camera as they are two very different things.

    Film and digital images are also very different when related to resolution!!

    Sorry for ripping this up a little but I feel strongly that as many people as possible should try to understand the relationships pixels in an image and ppi and the dpi of your printer and scanning resolutions.

    If I can help please let me know.

    Regards
    Jim
     
  12. EOS_JD

    EOS_JD TPF Noob!

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    What version of Photoshop do you use? Photoshop7 and higher uses the term ppi in the "image size" menu.
     

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